"bz" wrote in message
98.139...
"Mark D. Zacharias" wrote in
. net:



I will often sub in an external power supply and just see what gets hot.
No need to cut any foil runs at this point.

You need to be a bit careful to isolate the original supply because back
feeding some supplies can give misleading indications or damage the
supply.
For example, I have been told that some 3 terminal regulators are damaged
by
back feeding.

Also, you need to make sure you provide ALL + and - voltages in the right
order as some ICs do NOT like it when one or more supply voltages is
missing.

Cutting out the 5 volts is unlikely to do MORE damage than has already
occurred but feeding +5 without the -5, +12, -12 [or whatever]
could take out good ICs.

My FAVORITE tool is the 'Curve Tracer' that I build from a 1970's article
in
Radio/Television magazine.
It can spot 'sick' gates on an IC, and a lot of other problems 'in
circuit'.

It is similar to this one:
http://www.techlib.com/electronics/curvetrace.html

Mine is like the single range model. It is all built into a wall wart
except
for the test leads.
I clip my scope to resistor leads that come out through holes in the case.

Anyway, I find it very useful for locating bad transistors and bad gates
on
IC because the 'knee' gets rounded or is absent on bad junctions.

Often, you can test 'in circuit', but sometimes you will have to cut some
traces to isolate the junction from caps or resistors that are in parallel
with it.

A little practice with one of these, and a few 'reference junctions' to
compare with the curves you see, and you may love your curve tracer too.

Here is a modern version using your computer's sound card:
http://www.arrl.org/qst/2006/07/steber.pdf





--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

Never had a problem doing this - and in this case it's a single-ended 5 volt
supply. I've even added in a variable supply at the output of a regulator to
supplant the existing regulator while a unit is running. Just reduces the
current requirement on the existing regulator.

Found lots of shorted IC's and the like this way without having to trace a
power supply line all over a complex unit. About the only time this
technique hasn't worked in my experience is when there is a ZERO ohm short,
since some resistance is necessary in order for heat to be generated.


Mark Z.